I don't have much confidence in the SeeLevel gauge. I installed them on Juju's Westy and the results were pretty disappointing. Apparently the accuracy is affected by any deposits clinging to the interior of the tanks - ie: both black & grey.
My black tank gauge still works pretty good - not perfect but acceptable.
Your point is well taken though - trying to fit 20 lbs of sh** in a 10 lb sack.
Replacing the valterra gate valve on the bottom of the black tank is a difficult task - I've done it twice - so I know!
The first time was due to a leak between the gate valve and the tank. The problem turned out to be a design issue. They used a hard plastic spacer and copious amounts of that white paste that they used on many of the other fittings. I replaced it with a silicone rubber gasket I cut from a piece of thick sheet material. Since I already had the gate valve out I opted to replace it while as it.
Picture of the OEM hard black plastic spacer centered in the cutout from the thick silicone rubber sheet I used as a replacement.
The second time I replaced the gate valve was due to an unrelated issue. I had neglected to address a problem with the caulking around the perimeter of the shower pan. As a result some water got in under the shower pan and some smelly black mold had taken hold. An ounce of prevention could have prevented a pound of cure! Removing and replacing the shower pan basically results in having to R&R everything associated with the black waste plumbing!
I find it curious how many different opinions there are about the tank capacities
For example - here are three different Westy brochures with varying specs for the black tank capacity ie: 5.3 gal, 7 gal & 9.6 gal.
I'm not sure but I think 9.6 gal ( 36.3 liter ) is the correct value.